1. Field of Invention
The present invention relates to voltage regulators. More particularly, the present invention relates to low drop-out regulators implemented in a CMOS process.
2. Description of the Related Art
A voltage regulator is a device that produces an approximately constant output voltage. This output voltage will remain constant even if the load current changes. Similarly, the voltage regulator ensures that the output voltage remains constant for a variable input supply voltage. Accordingly, the regulator ensures that the output voltage is constant when at least one of the input voltage and the load current varies.
Referring to FIG. 1, a general block diagram illustrating a low drop-out regulator is presented. Typically, a low drop-out regulator 100 is used to maintain a low drop-out voltage. The drop-out voltage is the difference in voltage between an input voltage provided by an input supply 102 and an output voltage drawn by a load 104. It is desirable to maintain a low drop-out voltage in many instances to maximize the efficiency of a circuit, therefore minimizing voltage and power loss. This is particularly important in applications where the supply voltage is low (e.g., 3-12 volts). By way of example, for an input supply voltage of 5 volts and a drop-out voltage of 3 volts, the maximum output voltage that may be produced is 2 volts. This results in greater than 50% voltage and power loss. Accordingly, such voltage and power loss is typically minimized in low drop-out mode through the use of low drop-out regulators.
Low drop-out regulators are commonly fabricated using bipolar transistors. One beneficial characteristic of bipolar transistors is an approximately constant base-emitter voltage V.sub.BE. Thus, V.sub.BE remains constant regardless of the current through the transistor. Bipolar transistors are therefore ideal for use in applications such as the low drop-out regulator.
While low drop-out regulators may be fabricated using bipolar transistors, there are numerous advantages that may be provided by CMOS transistors. Currently, CMOS transistors are commonly used in the semiconductor industry in the fabrication of integrated circuits. Thus, it would be advantageous if a low drop-out regulator could be fabricated and integrated in such integrated circuits using a single process. Moreover, implementing an existing process would allow regulator designers to take advantage of existing research, reducing the design and fabrication costs. It would therefore be desirable if a low drop-out regulator could be fabricated using CMOS transistors rather than bipolar transistors.
Although the development of low drop-out regulators in the CMOS process would be beneficial, it is difficult to achieve a functionality equivalent to low drop-out regulators developed in the bipolar process. As described above, the V.sub.BE of a bipolar transistor remains constant. The equivalent of the V.sub.BE in a bipolar transistor is the ground-source voltage V.sub.GS in a CMOS transistor. However, since the V.sub.GS is not constant, the functionality of a low drop-out regulator cannot easily be duplicated using a CMOS process.
Various methods for fabricating low drop-out regulators in the CMOS process have been attempted. However, these regulators have been capable of functioning only when the driver transistor is in saturation region which means higher power loss. Accordingly, it would be desirable if a low drop-out regulator could be developed in the CMOS process that would be operable in both the linear and the saturation regions as well as provide reduced power consumption and operating costs.